GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 103 



monites, toward the close of the secondary period, has 

 been wonderfully sudden. 



The extinction of species has been involved in the 

 most gratuitous mystery. Some authors have even sup- 

 posed that, as the individual has a definite length of life, 

 so have species a definite duration. No one can have 

 marvelled more than I have done at the extinction of 

 species. When I found in La Plata the tooth of a horse 

 imbedded with the remains of Mastodon, Megatherium, 

 Toxodon, and other monsters, which all co-existed with 

 still living shells at a very late geological period, I was 

 filled with astonishment; for, seeing that the horse, since 

 its introduction by the Spaniards into South America, 

 has run wild over the whole country and has increased 

 in numbers at an unparalleled rate, I asked myself what 

 could so recently have exterminated the former horse 

 under conditions of life apparently so favorable. But 

 my astonishment was groundless. Professor Owen soon 

 perceived that the tooth, though so like that of the 

 existing horse, belonged to an extinct species. Had this 

 horse been still living, but in some degree rare, no 

 naturalist would have felt the least surprise at its rarity; 

 for rarity is the attribute of a vast number of species of 

 all classes, in all countries. If we ask ourselves why 

 this or that species is rare, we answer that something is 

 unfavorable in its conditions of life; but what that some- 

 thing is we can hardly ever tell. On the supposition of 

 the fossil horse still existing as a rare species, we might 

 have felt certain, from the analogy of all other mammals, 

 even of the slow-breeding elephant, and from the history 

 of the naturalization of the domestic horse in South 

 America, that under more favorable conditions it would 



